1. Field of the Invention
The present invention relates generally to a connection structure, and more particularly to a connection structure applied to a fan for connecting a metal member with a shaft by means of laser. The connection structure is applied to a fan with small torque or great torque to lower cost and enhance counterbalance of the fan and prevent the shaft from loosening.
2. Description of the Related Art
It is known that when a high-performance electronic apparatus works, a great amount of heat will be generated along with the operation of the electronic components inside the electronic apparatus. In general, a heat sink or a radiating fin assembly is disposed on the electronic components to enlarge the heat dissipation area and enhance the heat dissipation effect. The heat sink and the radiating fin assembly dissipate the heat simply by way of radiation so that the heat dissipation effect achieved by the heat sink and the radiating fin assembly is limited. Therefore, it is often necessary to additionally dispose a cooling fan to dissipate the heat of the heat sink or the radiating fin assembly and enhance the heat dissipation effect. Accordingly, the cooling fan has become a very important component in a thermal module.
In the conventional cooling fan, the fan impeller 10 and the shaft 12 are generally connected in two manners as follows:    1. As shown in FIG. 1A, the shaft 12 is integrally enclosed in the fan impeller 10 by means of injection molding. The shaft 12 is placed into a preset mold and then the fan impeller 10 is formed by means of plastic injection molding to integrally enclose the shaft 12. In the above structure, the plastic material has insufficient thermal deformation resistance and poor structural strength so that the fan impeller is subject to affection of high temperature or collision. As a result, the shaft is likely to deflect and the fan impeller is apt to deform. In addition, the shaft 12 is a cylindrical member, which is integrally enclosed in the fan impeller 10 by means of injection molding. Therefore, in the case that the structure is applied to a cooling fan 1 with great torque, the rotational torque of the fan impeller 10 will be greater than the connection torque between the shaft 12 and the fan impeller 10. Under such circumstance, the shaft 12 is very likely to loosen or even detached from the fan impeller 10 to cause damage and failure of the cooling fan 1.    2. As shown in FIG. 1B, the shaft 12 is fitted into a hole 131 of a copper sleeve 13 by means of pressing. Then the copper sleeve 13 with the shaft 12 is fitted into a fitting hole 141 of an iron casing 14 by means of pressing to form a hub 15. Multiple blades 16 are formed on an outer circumference of the hub 15 by means of plastic injection molding. The blades 16 are integrally connected with the hub 15 to form the fan impeller 10. The structure formed by means of pressing can improve the problems of deflection of the shaft and the deformation of the fan impeller due to high temperature. However, there is another problem existing in such structure. That is, the iron casing 14 is formed by means of a continuous molding process so that the total cost is relatively high. Therefore, the material cost for the hub 15 composed of the iron casing 14, the copper sleeve 13 and the shaft 12 by means of pressing is increased. Moreover, the shaft 12 is connected with the copper sleeve 13 by pressing and the copper sleeve 13 with the shaft 12 is connected with the iron casing 14 also by means of pressing. In this case, it is hard to control the concentricity of the shaft 12 and the copper sleeve 13. Also, it is hard to control the concentricity of the copper sleeve 13 and the iron casing 14. As a result, the counterbalance of the fan impeller 10 is poor. Furthermore, the copper sleeve 13 must have a considerable thickness for fixing the shaft 12. The thick copper sleeve 13 will occupy a considerably large internal space of the hub 15 of the fan impeller 10. Therefore, the internal use space of the fan impeller 15 is reduced.
According to the above, the conventional cooling fan has the following shortcomings:    1. The fan impeller made by means of injection molding has poor thermal deformation resistance and poor structural strength so that the shaft is likely to deflect and the fan impeller is apt to deform. Therefore, the structure can be hardly applied to a cooling fan with great torque.    2. The total cost for the fan impeller made by means of pressing is relatively high and it is hard to control the concentricity.    3. The counterbalance of the fan impeller is poor.    4. The internal use space of the fan impeller is reduced.